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hybrid vs grid-tie


stoic
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Sorry for being the powerforum rash the last few days. I'm learning, but clearly not fast enough.

My question today is grid-tie vs hybrid.

From my understanding, both pv modules and batterybank outputs DC.

My understanding of the difference between hybrid and grid tie is:

Grid-tie: it accepts power from PV and the grid, in most cases it pulls from PV, until such time the PV modules no longer supplies enough power and it then either balances it or pulls from the grid.

Hybrid: It accepts power from PV, grid and battery bank, and additionally offers charging capabilities for your batteries. When it comes to delivering power it can be set on when to use either of the three inputs.

For my purposes the hybrid inverter is the cleanest and simplest solution, but i am pondering the following setup:

Get a grid tied inverter, a charge controller and a battery bank. The battery bank is used PURELY for backup when the grid is down.

WARNING, professional drawing coming up.

 

image.png.49d71aaee4e557549e788ba511452750.png

 

The above picture depicts (for those that does not understand my brain) a manual version of the hybrid inverter.

I draw power from PV when sun is out, and occasionally charge the batteries when the sun is up (this happens manually), i can then manually switch to battery when the grid is down.

I have a fair amount of experience with a raspberry pi, and i can see myself hooking up a relay eventually and linking the pi to both the pv modules, the grid and the charge controller, and i should be able to eventually get the "manual switching" part automated.

But this whole system depends on my DC theory and that it is even at all possible to supply power to the grid tie inverter directly from a battery bank.

Yes, there is method to my madness here. I need to go three phase grid tied, and i want the battery backup PURELY for backup, it also allows me to experiment with different equipment on my 3 phase system without expanding a lot of money on hybrid systems every time my need changes.

Am i a a complete rookie when it comes to solar? I'll answer that... YES. But some logic applies, and i am putting it out there to.

 

 

 

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I drew two pictures of common hybrid topologies here. I actually had the Victron one wrong... there is no 400VDC bit... but such a topology can still be found in some places.

Yes, the panels are DC and so are the batteries. If you want to think about this in general, the key is (first) to remember that the peak voltage on the AC side is more like 340V (RMS voltage is roughly 70% of the peak). So one common method is to maintain two DC "pools", one at battery voltage, another one around the peak of what you're generating, for simplicity lets say 400VDC, with a buck/boost pipeline between them (so it can move power between these pools in either direction). If you have two such pools, you have a choice between connecting your PV panels to the high voltage pool, or the low voltage pool (aka directly to the batteries). There is no switching involved... energy moving into either pool can be sent either into the grid, or into the battery.

Then additionally modern inverters can be configured to limit the power it accepts from the PV panels, and then sometimes you can balance that with your loads, so that you end up being grid neutral.

A grid-tie inverter has a topology similar to the top picture, but it simply doesn't have the buck/boost leg going down to the battery. The MPPT part is essentially a buck/boost converter that directly maintains the required high-voltage pool from which it makes AC directly. It simply injects everything into the grid... again unless there is some kind of limiter that balances it with your loads. When the sun goes down... it simply switches off.

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